Oh Wow, Thank you so much for all your research and help. It's very much appreciated.
According to my amateur detective work, Rebecca Adams of Trewen was the mother of Grace Petherick - as per parish baptism register, 15 August,1892. Petherick may have been given as baby Grace's surname as a nod to impending legitimacy, or a middle name as an acknowledgement of the father. I believe this is most likely the Grace Adams who married James Harris at St Juliot, 15 July, 1818.
Interestingly, there is a record (photo image, Family Search) of the banns being called between Rebecca Adams of Trewen and William Petherick of Treneglos, in the month before Grace's birth . . . but there is no evidence of an eventual marriage.
Further, I have found a photo image record (Family Search) of the banns being called between (most likely our) Rebecca and Robert Hancock of Trewen in 1799/1800 (no date given) - but at least 9 months before the birth - and death - of baby Anne in October, 1800 - who was baptised as Ann Adams. No mention of the father again in the baptism records, and no evidence of a subsequent marriage between Rebecca and Robert. I found an image (Family Search) of a marriage licence between Peter Darton and a Rebecca Adams (1805) of the St Stephens by Saltash parish - and no evidence of the actual marriage again, so if that's our Rebecca, she does seem to be a recidivist fiancee . . . Interesting to speculate the story behind those details.
The family of Thomas Adams and Mary Cheley from St Winnows (Family Search) have a Rebecca who fits the necessary criteria - I'm just not able to find out what happened to her post-baptism.
I'm wondering if she's the sister of Charity Adams, and Joseph Adams (father of the Sophie Adams in the title of this post).
There's another Adams family from St Mellion - with a Rebecca and two other daughters who had illegitimate children - so there's another possibility.
It's very easy to mix up the members of the different families - with so many similar names, places and dates!
Many thanks again. You have been a great help. I shall continue to tease this out.